4. Quando na frase houverhad , were, should, pode-se omitir oif fazendo-se a inversão do verbo com o sujeito (para enfatizar a condição apresentada).Esta é uma construção formal, própria de linguagem literária.
Ex.:IfJohnhad arrived early, he would have seen the show.(HadJohnarrived early, he would have seen the show.).IfIwere rich, I would buy her a present.(WereI rich, I would buy her a present.)

5.Unless(se não; a menos que) pode ser usado em lugar de if not.
Ex.:
Bill won’t come to the party if you don’t invite him.(Bill won’t come to the party unless you invite him.)If Jane doesn’t study, she won’t pass. (Unless Jane studies, she won’t pass.)